1. Field of the Invention
The invention pertains to multiplicatively distorting data recording or transmission channels and has particular utility in high density digital recording systems.
2. Description of the Prior Art
It has been continuously desirable in digital mass data recording systems such as discs, drums and the like, utilizing magnetic recording media, to increase the density of the data recorded on the media. In this manner, the quantity of data stored in such systems may be increased. Conventionally, such systems utilize reading and writing heads that are aerodynamically supported a minute distance above the continuously rotating magnetic media. Prior art systems have to some extent been successful in increasing the data packing density by decreasing the head gap width, decreasing the head-to-media spacing, and decreasing the thickness of the magnetic recording media.
Additionally, present day systems utilize equalization circuits for decreasing inter-symbol interference and effectively increasing the recording frequency bandwidth thereby achieving higher data packing density. Although the prior art systems have been moderately successful in increasing the data storage density by the foregoing techniques, the maximum density heretofore achievable is significantly less than that predicted from theoretical considerations.
Typically, present day digital magnetic recording systems utilize the saturated flux recording technique wherein the flux in each data cell of the medium either remains saturated in the same direction as in the preceding cell or is switched to saturation in the opposite direction, to represent storage of a binary ZERO or binary ONE, respectively. Hence, under this system only one binary digit may be recorded in each data cell, the data cells being packed as densely on the medium as the foregoing prior art techniques will permit.
The prior art has contemplated that if instead of recording only one of two levels, i.e., one binary digit in each data cell, a larger number of levels were made possible, more than one binary digit could be stored in each data cell. For example, if it were possible to record a selected one of 8 discrete amplitude levels in a data cell, that cell would then be capable of storing three binary digits rather than one. Thus in addition to utilizing the aforedescribed prior art techniques for densely packing the data cells on the medium, a multifold increase in recorded information density may be achieved through multilevel recording.
Although contemplated by the prior art, multilevel digital recording has heretofore not been achievable because of multiplicative type gain variation distortions of the recording channels. Such gain variations may, for example, result from non-uniformity in the thickness of the magnetic coating. Variations in the distance between the write or read heads and the magnetic medium, caused for example by surface dimensional variations, also may result in gain variations of the channel. Because of the high data rate utilized in such systems, these gain variations which occur on a fractional-second time scale occur slowly relative to the recorded information rate. In such systems, although an anomalous gain change will normally be small from one recorded data item to the next, the gain excursions may be large in total effect so that distinguishing between one recorded level and another on data playback is rendered impossible. For example, if an aerodynamically supported read head should momentarily fly up due to a surface imperfection in the drum or disc, a large and unknown gain change will be multiplicatively imparted to the several hundred data words that may be read out during the anomolous condition. Thus previously contemplated multilevel recording systems would be unreliable in that gain variations would cause large blocks of data to be read out that bear no perceivable relationship to the corresponding data originally recorded. For the foregoing reasons, prior art digital recording systems have largely been limited to two oppositely saturated flux recording levels which naturally are differentially distinguishable from one another irrespective of the gain variations of the system. Hence, these prior art recording systems have been limited as to the information packing density achievable.